Differential regulation of p21ras activation in neurons by nerve growth factor and brain-derived neurotrophic factor

Neurotrophins activate the Trk tyrosine kinase receptors, which subsequently initiate signaling pathways that have yet to be fully resolved, resulting in neuronal survival and differentiation. The ability of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) to activate GTP binding to p21ras was investigated using cultured embryonic chick neurons. In both sympathetic and sensory neurons, the addition of NGF markedly increased the formation of Ras-GTP. The magnitude of the effect was found to depend upon the developmental stage, peaking at embryonic day 11 in sympathetic neurons and at embryonic day 9 in sensory neurons, times when large numbers of neurons depend on NGF for survival. Surprisingly, following the addition of BDNF, no formation of Ras-GTP could be observed in neurons cultured with BDNF. When sensory neurons were cultured with NGF alone, both NGF and BDNF stimulated GTP binding to Ras. In rat cerebellar granule cells, while the acute exposure of these cells to BDNF resulted in the formation Ras-GTP, no response was observed following previous exposure of the cells to BDNF, as was observed with sensory neurons. However, this desensitization was not observed in a transformed cell line expressing TrkB. In neurons, the mechanism underlying the loss of the BDNF response appeared to involve a dramatic loss of binding to cell-surface receptors, as determined by cross-linking with radiolabeled BDNF. Receptor degradation could not account for the desensitization since cell lysates from neurons pretreated with BDNF revealed that the levels of TrkB were comparable to those in untreated cells. These results indicate that in neurons, the pathways activated by NGF and BDNF are differentially regulated and that prolonged exposure to BDNF results in the inability of TrkB to bind its ligand.